Electronic converters are widely used in power-generating applications, such as, for example in wind power generators. The desired requirements of a converter include obtaining an output current with the best THD (Total Harmonic Distortion) possible and obtaining a high voltage to minimize conduction losses.
Today, most manufacturers of electronic conversion stages are trying to increase the power thereof by increasing the number of voltage levels in the output voltage. This not only increases the power used but it also improves the quality of the current delivered, greatly reducing the harmonic content.
The two previous objectives can be achieved by means of using multilevel converters. These converters are capable of working with different voltage levels in order to obtain an output current with the best THD possible, i.e., with the best wave quality possible. They are also capable of increasing the working voltage which is a desirable characteristic in order to obtain fewer losses.
A large amount of multilevel conversion topologies like those described in patent documents EP0555432A1, EP1051799B1, EP1673849A1, EP1815586A1, EP2080262B1, is known in the state of the art even though most of them involve an excessively complicated and expensive electromechanical design.
A multilevel converter providing the previous characteristics is the converter described in EP0555432A1. This converter is formed by n cells made up of a capacitor and two semiconductors that are connected in series, one after another. The increase of the voltage levels available is done by means of adding the voltages to or subtracting the voltages from the capacitors. However, this converter has the problem of having a large size and high cost which greatly complicates the electromechanical design. The need of this large size limits the number of levels attainable by this converter since there will come a time when the volume required makes it non-viable.
EP1051799B1 proposes a multilevel converter formed by a converter of the NPC (Neutral Point Clamped) type in which clamp diodes are replaced with ANPC (Active Neutral Point Clamped) controlled semiconductors in the output stage of which capacitors are intercalated for the purpose of increasing available voltage levels. The main problem of the converter described in EP1051799B1 is that in order to obtain an n-level converter, it is necessary to serialize (n−1)/2 controlled semiconductors.
EP1673849A1 describes a multilevel converter formed from the serialization of several switching units, each of these made up of two controlled semiconductors arranged in series through a capacitor. The main problem that this converter has is that in switches there are parasitic inductances causing unwanted voltage surges such that it reduces the current capacity and the service life of the semiconductors. EP1815586A1 has tried to mitigate this phenomenon by means of a circuit using semiconductors and additional voltage limiter circuits. These additional elements increase the cost and size and they add complexity to the converter.
EP2080262B1 proposes a converter similar to that of EP1673849A1 in which additional semiconductors are introduced for the purpose of reducing the number of capacitors necessary and increasing the number of levels. These additional semiconductors also increase the cost and size and they add complexity to the converter.